Maria Jacinto, with her husband, Aristeo, and one of their five children, speaks only Spanish. "When my skin turns white and my hair turns blonde, then I'll be an American," she says.(By William Branigin  The Washington Post)

OMAHA  Night is falling on South Omaha, and Maria Jacinto is patting tortillas for the evening meal in the kitchen of the small house she shares with her husband and five children. Like many others in her neighborhood, where most of the residents are Mexican immigrants, the Jacinto household mixes the old country with the new.

As Jacinto, who speaks only Spanish, stresses a need to maintain the family's Mexican heritage, her eldest son, a bilingual 11-year-old who wears a San Francisco 49ers jacket and has a paper route, comes in and joins his brothers and sisters in the living room to watch "The Simpsons."

Jacinto became a U.S. citizen last April, but she does not feel like an American. In fact, she seems resistant to the idea of assimilating into U.S. society.

"I think I'm still a Mexican," she says. "When my skin turns white and my hair turns blonde, then I'll be an American."

In many ways, the experiences of the Jacinto family are typical of the gradual process of assimilation that has pulled generations of immigrants into the American mainstream. That process is nothing new to Omaha, which drew waves of Czech, German and Irish immigrants early this century.

But in the current immigration wave, something markedly different is happening here in the middle of the great American "melting pot."

Not only are the demographics of the United States changing in profound and unprecedented ways, but so too are the very notions of assimilation and the melting pot that have been articles of faith in the American self-image for generations. E Pluribus Unum (From Many, One) remains the national motto, but there no longer seems to be a consensus about what that should mean.

There is a sense that, especially as immigrant populations reach a critical mass in many communities, it is no longer the melting pot that is transforming them, but they who are transforming American society.

American culture remains a powerful force  for better or worse  that influences people both here and around the world in countless ways. But several factors have combined in recent years to allow immigrants to resist, if they choose, the Americanization that had once been considered irresistible.

In fact, the very concept of assimilation is being called into question as never before. Some sociologists argue that the melting pot often means little more than "Anglo conformity" and that assimilation is not always a positive experience  for either society or the immigrants themselves. And with today's emphasis on diversity and ethnicity, it has become easier than ever for immigrants to avoid the melting pot entirely. Even the metaphor itself is changing, having fallen out of fashion completely with many immigration advocacy and ethnic groups. They prefer such terms as the "salad bowl" and the "mosaic," metaphors that convey more of a sense of separateness in describing this nation of immigrants.

"It's difficult to adapt to the culture here," said Maria Jacinto, 32, who moved to the United States 10 years ago with her husband, Aristeo Jacinto, 36. "In the Hispanic tradition, the family comes first, not money. It's important for our children not to be influenced too much by the gueros," she said, using a term that means "blondies" but that she employs generally in reference to Americans. "I don't want my children to be influenced by immoral things."

Over the blare of the television in the next room, she asked, "Not all families here are like the Simpsons, are they?"

Among socially conservative families such as the Jacintos, who initially moved to California from their village in Mexico's Guanajuato state, then migrated here in 1988 to find jobs in the meatpacking industry, bad influences are a constant concern. They see their children assimilating, but often to the worst aspects of American culture.

Her concerns reflect some of the complexities and ambivalence that mark the assimilation process these days. Immigrants such as the Jacintos are here to stay but remain wary of their adoptive country. According to sociologists, they are right to be concerned.

"If assimilation is a learning process, it involves learning good things and bad things," said Ruben G. Rumbaut, a sociology professor at Michigan State University. "It doesn't always lead to something better."

At work, not only in Omaha but in immigrant communities across the country, is a process often referred to as "segmented" assimilation, in which immigrants follow different paths to incorporation in U.S. society. These range from the classic American ideal of blending into the vast middle class, to a "downward assimilation" into an adversarial underclass, to a buffered integration into "immigrant enclaves." Sometimes, members of the same family end up taking sharply divergent paths, especially children and their parents.

The ambivalence of assimilation can cut both ways. Many native-born Americans also seem to harbor mixed feelings about the process. As a nation, the United States increasingly promotes diversity, but there are underlying concerns that the more emphasis there is on the factors that set people apart, the more likely that society will end up divided.

With Hispanics, especially Mexicans, accounting for an increasing proportion of U.S. population growth, it is this group, more than any other, that is redefining the melting pot.

Hispanics now have overtaken blacks as the largest minority group in Nebraska and will become the biggest minority in the country within the next seven years, according to Census Bureau projections. The nation's 29 million Hispanics, the great majority of them from Mexico, have thus become the main focus for questions about how the United States today is assimilating immigrants, or how it is being transformed.

In many places, new Hispanic immigrants have tended to cluster in "niche" occupations, live in segregated neighborhoods and worship in separate churches. In this behavior they are much like previous groups of immigrants. But their heavy concentrations in certain parts of the country, their relatively close proximity to their native lands and their sheer numbers give this wave of immigrants an unprecedented potential to change the way the melting pot traditionally has worked.

Never before have so many immigrants come from a single country  Mexico  or from a single linguistic source-Spanish-speaking Latin America. Since 1970, more than half of the estimated 20 million foreign-born people who have settled in the United States, legally and illegally, have been Spanish speakers.

Besides sheer numbers, several factors combine to make this influx unprecedented in the history of American immigration. This is the first time that such large numbers of people are immigrating from a contiguous country. And since most have flowed into relatively few states, congregating heavily in the American Southwest, Mexican Americans have the capacity to develop much greater cohesion than previous immigrant groups. Today Hispanics, mostly of Mexican origin, make up 31 percent of the population of California and 28 percent of the population of Texas.

In effect, that allows Mexican Americans to "perpetuate themselves as a separate community and even strengthen their sense of separateness if they chose to, or felt compelled to," said David M. Kennedy, a professor of American history at Stanford University.

To be sure, assimilation today often follows the same pattern that it has for generations. The children of immigrants, especially those who were born in the United States or come here at a young age, tend to learn English quickly and adopt American habits. Often they end up serving as translators for their parents. Schools exert an important assimilating influence, as does America's consumer society.

But there are important differences in the way immigrants adapt these days, and the influences on them can be double-edged. Gaps in income, education and poverty levels between new immigrants and the native-born are widening, and many of the newcomers are becoming stuck in dead-end jobs with little upward mobility.